Development of biodiesel skeletal mechanisms for kinetic combustion modeling

This paper reports the developmental work of skeletal biodiesel surrogate mechanisms specifically for integration with computational fluid dynamics (CFD) solvers. The fuels of interests here were methyl esters of coconut, palm and soybean. Their combustion kinetics were collectively represented by a detailed mechanism with appropriate components of a biodiesel surrogate (methyl decanoate/methyl-9-decenoate) and a diesel surrogate (n-heptane). As a result of computational complexity induced by the detailed mechanism with 3299 species and 10806 reactions, several mechanism reduction methods were employed such as directed relation graph, isomer lumping and temperature sensitivity analysis. Three different skeletal mechanisms, with the inclusion of low- and high-temperature chemistries, were built as a result of different governing reaction pathways in each fuel. The resulting skeletal mechanisms for coconut, palm and soybean contained 92 species and 337 reactions, 93 species and 339 reactions, and 82 species and 310 reactions, respectively. Each skeletal mechanism was validated under 48 shock tube conditions, using zero-dimensional (0D) closed homogenous reactor model in CHEMKIN-PRO. Errors of less than 35% were found when the ignition delay times of each proposed biodiesel skeletal mechanism were compared to that of the detailed mechanism. Good agreement of target species profiles prediction between the skeletal mechanisms and the detailed mechanism was also obtained. An overall savings of 30% in computational runtime was achieved for the developed biodiesel skeletal mechanisms. These results proved that the proposed skeletal mechanisms are able to provide reasonable description of the combustion kinetics for biodiesel fuels.